Better but not a great design. Too many directional hard bends create turbulance.. Pipes need to be straight and bends need to be gradual. X pipes like a 45 degree angle.
To see a good v12 exhaust design we can look to Aston Martin and their V12, They coaxed 450HP from a 5.9 V12 and the exhaust design is free flowing and reasonably quiet. Aston used a pressure wave termination box as show below. No possibility of reversion.

Too many hard bends create turbulence that negate any benefit to an X pipe.





Aston V12 exhaust center section. All silencers are baffled and straight through. Refined enough for an Aston Martin.
This is the design to copy. Center section is a merge box or pressure wave termination box.

DB7 V12 exhaust

Right way to do an X pipe with gradual bends and X angle very close to 45 degrees. No chance of reversion and passing pulse can create a draw on opposing side. Less effective on a V12 but proven very beneficial on a V8 where pulses have more separation time on V8 which tend to have some overlap in the cam.

 

Depends what you're after.

The benefit for me, and the sole purpose of incorporating the X pipe, was improved exhaust tone. With lower restriction mufflers the V12 exhaust note is very odd....unless an H or X pipe is used.

Cheers
DD

 

That was a good read. Thanks! All the info on their site is excellent.

I imagine that there isn’t room for a proper set of equal-length 6:1 headers in an XJS no matter how creatively pretzeled.

The good thing is that iron log manifolds are fine, and while properly engineered extractors can be beneficial, these 3:1, 2:1 headers won’t hurt. Their statement that tightly twisted pipes have no significant negative effects on the exhaust is well supported by the picture of the F1 exhaust knot. That makes me feel better about this proposed routing!

Pipe size is already set in stone. I haven’t taken measurements of the primaries or secondaries, but these end with 2.5” flanges. Side pipes are 3”. Given the final merge into dual pipes this system will be very large on the exit end! I can’t imagine that this set of headers ( AJ6 manufactured ) will kill low end torque even if I ran them open, but we will see.

There seems to be a wide gulf between the stock-is-good-enough-leave-it-alone and perfected-best-possible-system. I’m fine being somewhere in the middle of that.

 

If I may resume to paraphrase Mr. Bywater, I believe that he is saying in essence that: Up to 5,000 rpm neither the exhaust nor the intake is the restriction on the HE engine. Any restriction is a factor of the combustion chamber design. But this is not really a fault, remembering that the designers' aim was a refined adequately powerful road car engine with decent fuel economy.

I was very interested in ICS America's exhaust points about the AM V12, and would like to hear more if he has the time. That engine, we must all remember, is a 4 valve pentroof design, much more modern than our V12s, and is two Ford 2.5 litre V6s joined together. That Ford V6 being one of the best engines Ford have ever made. What I do know for sure is that if you use the HE's performance and use the performance of a much more modern engine of similar capacity, the real world difference in fuel consumption will be surprisingly small.

 

The AM / Ford V12 made 450HP from 5.9 littres while meeting modern emissions requirements with no variable valve timing tricks. No doubt the head design was a key part of that and that displacement to power ratio is still impressive nearly 20 years on. One of the things AM / Ford did in the exhaust was to use rather large catalytic converters. They are over sized and long and the floor pan had to be modified significantly to accommodate them, For those that dont know the AM DB7 and XJS use the same floor pan. The floor pan modifications take up so much space that the foot well is very cramped. On the AM V12 the pipes are 2.5 inches and then neck down to 2.25 by the time they get to the over the axle pipes.

It has been proven on the dyno in the V8 world that the header extension is extremely important for something like 30 to 50 HP on the dyno but that's with a performance 'overlappy' cam. With a stock cam the effect would be there but much less. The header extension area is where the cat converters live on the XJS and most other modern cars. Lengthening this area with a straight pipe is something many auto enthusiast can do on their older cars that are no longer emissions controlled. On new cars they managed to get some of the header extension back by moving the catalytic converter even further upstream and nearly on the exhaust manifold creating a longish smooth header extension effect after the fact.

 

long down pipes, are good for torque , 2.5" all way from exhaust manifolds(that have been ported and Ceramic coated in and out), mufflers are 2.5" straight thru, and my cams are wide overlap, CRANE Cams!

Hwy works for me,pic!
ron

 

Ronbros, whats your piping under the car look like? Downpipes into 90s and straight to the side?

 

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Hi Jig,, yes pretty much like that!
custom all 2 1/2" piping, blended at the manifold outlets, angle slightly rearward, left side is around 2 ft longer than right side going under the space near the flywheel, but some say it extends the torque curve giving a longer curve,(maybe slightly lower).

cant prove it by me tho!

all told the system is around 6 feet long on right,, and 8 ft on right side including mufflers, all EXIT from a 5" deep bass outlet,, the sound is incredible ,cams rough idle NICE, and 12 cylinders at 6500/7000 rpm is wailing scream! OK maybe not for everyone but i get a lot of thumbs up, which OK by me! muscle car guys LOVE it!
ron

 

Full disclosure: I don't have headers on any of my XJSs. But I do have SS headers on my V12 XKE. Not sure they help, yet I assure you, I would do it again over and over if for no other reason than the sound.